Multiple thymine dimer excising nuclease activities in extracts of human KB cells

Crude extracts of human KB cells grown in suspension culture contain enzyme activity that catalyzes the preferential excision of thymine-containing pyrimidine dimers from UV-irradiated E. coli DNA specifically incised adjacent to dimer sites. Fractionation of KB cell crude extracts reveals the presence of three such activities with distinct affinities for both DEAE-cellulose and phosphocellulose. One of the activities (activity B) is distinguished by its s 20,w (2.6) and isoelectric point (9.0) from the other two (activities A and C) which have similar s 20,w's (3.0-3.2) and isoelectric points (6.0). All three differ in their extent of stimulation by divalent cation and inhibition by NaCl or a sulfhydryl group inhibitor. These results indicate that multiple 5' leads to 3' dimer excision nuclease activities exist in human cells; however, there is as yet no direct evidence that these enzymes are functional in nucleotide excision repair in vivo.